Barcelona stumbled to a new setback in a troubled week as Inaki Williams' header in added time dumped last season's beaten finalist out of the Copa del Rey.
A hard-fought quarterfinal appeared destined for extra time until Williams applied enough of a touch to Ibai Gomez's inswinging cross from the right flank to send the ball beyond Marc-Andre ter Stegen and into the net.
The winner in the third additional minute saw Athletic through to the last-four and, with Real Madrid surprisingly beaten at home by Real Sociedad just hours earlier, it means neither of Spain's two heavyweight clubs are left standing.
Lionel Messi, who had made the headlines in the lead-up to the game for criticising director of football Eric Abidal, had Barca's best chance, only to be denied by Unai Simon's outstretched left leg late in the second half.
The Athletic goalkeeper had also denied substitute Antoine Griezmann but, for the first time in seven seasons, the Catalan club will not be involved in the final of the competition.
Abidal had stoked the flames with comments in a media interview where he said some players had not worked hard enough under previous boss Ernesto Valverde, yet the visitor did not demonstrate a lack of fight in Bilbao.
Messi was one of six Barca players to see yellow, booked for a sliding challenge on Raul Garcia in a first half that saw Ansu Fati go close, the teenager seeing an on-target effort on the turn blocked by the retreating Unai Nunez.
However, it was Athletic who carried the greater threat before the break. Williams correctly had a close-range finish from a corner ruled out for offside, while Dani Garcia saw a rising shot drift narrowly wide after Barca lost possession near its own penalty area.
Barca did improve in the second half yet failed to capitalise on its opportunities as it controlled possession.
The normally reliable Messi was unable to provide the finish to Arthur’s throughball, and the let-off allowed Athletic to snatch the win thanks to Williams' dramatic intervention at the other end.
What does it mean? Barca a fading force
It has been a turbulent few months for the reigning LaLiga champion. A change in coach, a key injury to Luis Suarez and now a disgruntled superstar, with Messi responding to Abidal's words by telling the Frenchman to name those individuals who failed to pull their weight. Airing dirty linen in public is not a good look. Quique Setien described the job as beyond his "wildest dreams" following his appointment – it may be turning into nightmare after this latest setback.
Heads, you win!
Williams was forced to feed off scraps after the interval, yet he had the presence of mind to get across the front of Clement Lenglet and reach Gomez's teasing delivery. However, it should not be forgotten that it was Simon's saves that set up the opportunity for the striker to be the hero.
Injury setback in defence
As if the result was not bad enough, Barcelona saw Gerard Pique forced to come off in the 80th minute with an apparent groin injury. His departure will be a concern, particularly with the first leg of its Champions League last-16 tie with Napoli coming up at the end of the month.
Barcelona will hope for better fortunes when it heads to Real Betis for the late game on Sunday, with Athletic in earlier action as it visits a Real Sociedad side fresh from knocking Madrid out of the cup.